The classical view of the gap genes is that they are expressed in domains [1]}. Implicitly this suggests that fine scale variations in the concentration of these molecules is not important; rather all that matters whether expression is on or off. The quantitative version of this idea is that subsequent events are sensitive to whether expression levels are above or below a threshold, corresponding to whether a cell is inside or outside an expression domain. We know that such simple thresholding omits a lot of the information that gap gene expression levels carry about position along the anterior-posterior axis [2]}.
<FIGURE>